The technology for manufacturing circuits comprising components mounted on printed circuit boards has evolved continuously during the past fifty years keeping pace with the evolution of discrete semiconductor devices to present high density integrated circuits on a chip.
The first boards were simply plastic phenolic boards with printed wiring to which discrete components were hand soldered. As chips holding integrated circuits were developed, the technology of "plated-through-holes technology appeared.
The dip is a chip imbedded in a plastic block enclosure generally about the size of a postage stamp. The block also contains leads extending from contact with the chip to outside the block so that the integrated circuit resembled a centipede including the block and one row of legs (legs) along one edge of the block opposite a second row of legs along an opposite edge.
The relentless demand for greater component density on the board and reduced cost led to the development of "Surface Mount Technology" which has either replaced or been used in combination with plated through hole technology. "Surface mounting" is the name of the process in which a packaged IC or other component is physically mounted on the surface of the PWB instead of inserting leads into plated holes through the PWB.
The steps in the method of the surface mount process (prior art) are described as follows:
In step 1, components (Integrated Circuits, capacitors, resistors, etc.) are provided in which the leads are short and "tucked" against the component or otherwise bent (during the manufacturing process) so that a relatively broad surface of each lead is turned in a common direction facing the surface of the printed wire board. The leads are pretinned.
In step 2, a plastic board is provided with a wire circuit printed on at least one side.
In step 3, the board is cleaned using one of several processes depending on the condition of the board i.e., degree of oxidation, etc.
In step 4, a "foot pad" pattern of patches is deposited on the board wherein each foot pad ultimately becomes an area to which a lead of a component is soldered. The foot pad is a patch of solder paste. The foot pad pattern is typically deposited by a screening method such as is used in printing processes.
In step 5, the components are placed at the required locations on the screen with each lead of the component in contact with the appropriate patch. This is commonly done with "pick and place" machines which is programmed to place the component in the exact required location on the board.
In step 6, the board with components is heated causing the solder paste to melt and then securely attach the lead to the solder patch (after, of course, the solder patch solidifies). Special heating techniques have been developed which offer advantages particularly for the solder reflow step in the surface mount process. One such process is vapor phase heating in which the board carrying the components for reflow are first preheated in a vaporous environment to one temperature for the purpose of eliminating thermal shock in a second step which is the step of a higher temperature vaporous environment. In the second step, the heated vapor condenses on the surface of solder paste because the solder paste is at a lower temperature when it is first inserted into the higher temperature chamber. When the vapor condenses on the board, the vapor gives up its heat of condensation to the solder and melts the solder.
A requirement for all of these processes is that each board must be supported as it is carried through the various steps from one process station to the next. This is generally accomplished in the industry by mounting the board on a support fixture. Because the requirements for each station varies, general practice is to provide a fixture that is designed for that specific station. Therefore, when the board is moved from one station to the next, it is placed on a fixture designed specifically for that station. This frequent transfer from one fixture to another is time consuming, subject to accidents or misalignments causing errors, introduces additional costs in terms of a requirement to provide more fixtures, provide storage space and accounting activity to keep track of the fixtures. Therefore there is strong interest in reducing the number of fixtures that are used throughout the process.
A particularly critical step is the stencil printing step where it is required to apply a squeegee to force the stencil into contact with the board and to repeat this operation many times (board after board). This step is made even more difficult when pressure is applied to the screen which stretches the screen each time the screen is used. Eventually the screen becomes permanently stretched so that registration between features is diminished leading to inaccuracies.
In the "pick and place" (of components) operation, the apparatus required to place the part would be less expensive if the board could simply slide horizontally into position under the placement head rather that having to include the step of withdrawing the head from the level of the surface of the board in order to avoid hitting the frame when the frame carrying the board is slid into position.
Another important expense is the requirement to have various sizes of fixturess--each size matching a size requirement for a board.
Another characteristic that determines the degree to which a fixture satisfies the requirements of all of the steps in the manufacturing process is the ability of the fixture to not become a heat sink during the solder reflow step and the ability of the fixture not to warp as a result of exposure to heat.
Another important requirement is to maintain precise registration in the placement of each component during the pick and place operation with respect to the screen printed pattern and this requirement is a major factor in determining the achieveable density in manufacturing the printed circuit.
A number of disclosures have appeared related to fixtures for manufacturing of printed circuit boards.
For example; U.S. Pat. No. 5,785,307 to Chung discloses a frame for supporting a printed circuit board. Spring loaded clamps are mounted around the area with fingers that may e oriented out over the area to retain the board. The board is released when the stool is rotated so as to orient the finger out of contact with the board.
Japanese Patent 2-153591 to Okuno discloses a method using conductive paste to repair a printed circuit board.
Japanese Patent 48238 to Seisakusho discloses a one touch security arrangement for securing a board to a table.
None of these inventions disclose a single board that addresses all of the problems discussed above.